1. Field of the Invention
The present invention relates to a drill, and, in particular, a rock drill including a stem having at least one discharge groove helically circumscribing the stem, and a drill head provided at one end of the stem and having at least two hard metal bits projecting radially beyond a drill head circumference and drilling dust grooves extending at least partially between the bits and opening into the at least one discharge groove.
2. Description of the Prior Art
For forming bores in concrete, brickwork, stone and the like, drills which have bits formed of a cutting metal, in particular hard metal, are used. The drills are usually inserted in hand-held tools which, in addition to a rotary drive, include a hammer mechanism for impacting the drill. The impacts blows are applied to the drill shank, which is received in a chuck of the drilling tool, and are transmitted through the drill stem and the drill head into a constructional component. In this way, the constructional component is shaved, chiseled, and chipped away. The drill should be able to meet high requirements with regard to drill feed, necessary force expenditures and have as long as possible service life.
Prior act discloses a number of drills which are characterized, under normal operational conditions, by satisfactory feed characteristics and force expenditures. Also, under normal operational conditions, their service life is also acceptable. Such a drill is disclosed, e.g. in European Publication EP-A-0 669 468. The drill has a stem with a shank provided at an end of the stem and adapted to be received in a hand-held tool, and a drill head provided at a stem end opposite the shank. The drill head has four bits which are provided in a star-shaped insert formed of a hard material. The bits are separated by drilling dust grooves which open in two discharge grooves for discharging the drilling dust and which helically circumscribe the stem. The known drill is capable of meeting the necessary requirements, in particular of a professional user, at small and medium bore diameters. However, for the use of the drill for drilling large diameter bores, it needs improvement. In particular, during drilling a bore in a reinforced concrete, the drill can encounter a reinforcing metal and be hooked over the reinforcing metal. The danger of a drill head being hooked over the reinforcing metal increases with an increase of a diameter of a to-be-drilled bore.
Accordingly, an object of the present invention is to eliminate the drawbacks of the prior art drills and provide a drill with which a danger of the drill being hooked over the reinforcing metal during drilling bores in a reinforced concrete is reduced even during drilling bores having a large diameter.
Another object of the present invention is to provide a drill having good feed characteristics at a reduced expenditure of force, and having an increased service life.
These and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a drill, in particular a rock drill, including a stem having at least one discharge groove and a drill head provided at the end of the stem opposite the stem end provided with a shank and having at least two hard metal bits projecting radially beyond a drill head circumference. The drill head further includes drilling dust grooves extending at least partially between the bits and opening into the at least one discharge groove. The drill head also has, at least in a region of the at least two hard metal bits, an extended circumferential profile projecting radially beyond a circumference of the stem.
The formation of the drill head, in the region of the bard metal bits, with an extended circumferential profile reduces the extent of the radial projection of the bits with respect to the drill head. This structure counteracts the tendency of the projecting bits to hook over the reinforcing metal, without increasing in any substantial way the friction between the drill head and the bore wall. In addition, in particular when the hard metal bits, which are arranged in a circumferential region of the drill head, are formed as plate-shaped inserts, the inserts have a better support in their bottom region. A better embedding of the bits in the drill head reduces the danger of their loosening or falling out. It also increases the service life of the drill.
Preferably, the radially extended circumferential profile of the drill head projects radially beyond the stem circumference by a distance that amounts from about 2% to about 10% of the stem radius. With these values of the radial projection of the circumferential profile in question, a noticeable better support for the bits is provided, without increasing the friction between the drill and a bore wall.
In an advantageous embodiment of the present invention, the radially extended circumferential profile of the drill head extends on opposite sides of a side guide region of the hard metal bit over an angular region from about 5xc2x0 to about 30xc2x0. The radial projection of the side guide region improves the retention of the drill in the reinforcing metal. The bits, which are generally formed as hard metal plates, are better embedded in the material of the drill head. As a result, the embedded hard metal bits are less sensible to the action of transverse forces acting in the circumferential direction upon removal of a bore wall material.
In case of the bits being formed as hard metal inserts, it is advantageous when the radially extended circumferential profile has an axial extension corresponding at least to an axial length of the bottom region of the hard metal bit embedded in the drill head. This insures the support of the bottom region of the bit along its entire axial extent. This prevents hooking of the bit over the reinforcing metal. Rather, due to the embedding of a bit in accordance with the present invention, the hard metal bit is smoothly guided along the reinforcing metal.
The friction resistance during the operation is reduced and the drill feed can be improved when the drill head circumferential profile in a region of the drilling dust grooves is spaced from a drill central axis by a distance smaller than a distance the guide region of a hard metal bit is spaced from the drill central axis.
In order to be able to reliably prevent hitting and hooking of edges, which are provided based on design considerations, upon encounting the reinforcing metal, advantageously, a projection of the radially extended circumferential profile is curved toward the leading edge of an adjoining drilling dust groove in the rotational direction of the drill. In this way, one of the causes of a possible hooking of the drill is eliminated. In case of encounting of a reinforcing metal, the drill continuously slides there along. By rounding the leading, in the rotational direction, edges, the sliding tendency is insured. This also insures a quasi-continuous transition of the projection into side guide region of the bit, and the danger of hooking of an edge on the drilling head with the reinforcing metal is substantially reduced.
In order to reduce the frictional forces acting on the wall of a drilled bore, advantageously, the drill head is formed so that its diameter in the entire side guide region of the hard metal bits, is larger than the diameter of the drill stem.
According to one of the embodiments of the present invention, the drill head has three hard metal bits which project beyond the drill head circumference. Advantageously, the hard metal bits are arranged symmetrically in a star-shaped pattern and are separated by respective, axially extending, drilling dust grooves.
According to one of the preferred embodiment of the present invention the hard-metal bits are formed as plate-shaped inserts inserted in an end surface of the drill head, with each plate-shaped insert having a side guide region which is so inclined to an axial extension of the radially extended circumferential profile that a free end of the plate-shaped insert is radially spaced from the drill head a distance larger than a distance an enbedded bottom portion of the insert is spaced from the drill head.
In this embodiment of the inventive drill, the cutting edges of the hard metal plates extend from respective bottom portions of the plates, which are completely embedded in the radially extended circumferential profile of the drill head, in a direction of respective free ends radially outwardly. This insures a sufficient radial projection, of the plate-shaped bits while simultaneously insuring, a good radial support of the bits. As a result, the frictional forces applied to the bore wall can be reduced.
For manufacturing reasons, in a further embodiment of the present invention, the entire drill head is formed of a hard metal. The higher costs of a drill according to this embodiment are more than compensated by the possibility to form the drill head with any arbitrary selected shape. Also, such a drill head is more easily handled and is more easily mounted.
The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings.